Electrochemical dissolution

ABSTRACT

A process for the electrochemical dissolution of a metallic structure having a plurality of electrically conducting components comprises utilising the structure as a sacrificial electrode in an electrochemical cell so as to dissolve at least part of the strcuture. The process is characterised in that, prior to the use of the structure as a sacrificial electrode, molten metal is allowed to solidify about the structure so as electrically to connect together the components.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a U.S. national phase application of PCTInternational Application No. PCT/GB99/03427, having an internationalfiling date of Oct. 22, 1999 and claiming priority to Great BritainApplication No. 9823046.9 filed Oct. 22, 1998. The above PCTInternational Application was published in the English language and hasInternational Publication No. WO 00/25321.

FIELD OF THE INVENTION

The present invention concerns electrochemical dissolution. By way ofexample, the present invention relates to the reprocessing of irradiatednuclear fuel and, by way of particular example, to the dissolution ofnuclear fuel assemblies. Reference will be made hereinafter to thedissolution of nuclear fuel assemblies but it should be understood thatthe present invention has application to the dissolution of otherstructures including those which are radioactive as well asnon-radioactive structures.

BACKGROUND OF THE INVENTION

A nuclear fuel assembly typically includes a plurality of fuel rods orpins which are assembled together within a skeleton of grids. Each fuelrod or pin may typically be in the form of a carrier tube made of asuitable material such as the zirconium metal based alloy known asZircaloy. Within the carrier tube is located a “stack” of nuclear fuelpellets.

In a known commercial method of reprocessing of irradiated (spent)nuclear fuel, the pins of an assembly are chopped up prior todissolution of the uranium dioxide fuel pellets in nitric acid. The pinsmust be chopped up to expose the pellets to nitric acid because the bulkzirconium alloy is resistant to attack by nitric acid, as is an oxideskin or film which is present on the irradiated zirconium alloy. Thechopping up of the pins is undesirable because it requires mechanicalapparatus which becomes subject to serious wear and therefore requiresrelatively frequent repair. It will be appreciated that there aredifficulties in repairing machinery which is used for the processing ofradioactive material.

In a period between the 1950s and 1970 considerable experimental workwas carried out on the electrochemical dissolution (“ECD”) of complete(unchopped) fuel pins. A review of the development of ECD up to 1970 canbe found in V P Caracciolo and J H Owen, progress in nuclear energy,Series III. Vol IV. Pergamon Press 1970, pp 81-118. The principle of ECDis that a fuel pin is placed in nitric acid and a potential differenceis applied between the fuel cladding and the nitric acid surrounding it.If this potential is large enough then the inert nature of the claddingis overcome and it becomes reactive to the nitric acid.

Caracciolo and Owen describe a process in which the fuel pins are placedin wedge-shaped baskets whose sides taper towards each other towards thebottom of the basket. The basket is placed in nitric acid and isconnected to a power supply. However, as the dissolution process is anoxidative one, contact between the metallic basket and the oxidisingassembly is lost resulting in cessation of the process. Furthermore, theprocess has only been demonstrated on stainless steel clad fuel andwould not work with Zircaloy clad fuel because the oxide film preventselectrical contact between basket and fuel assembly.

In addition to the problem posed by the pre-existing oxide film, thereis also the problem that, due to the large number of fuel pins in anassembly, it is difficult to form adequate electrical contact to all thepins during the process. A pressurised water reactor (PWR) assembly caninclude up to 324 pins in an 18×18 matrix assembly. Attempts made toform direct electrical contact with the fuel assembly have all faileddue to the problem of forming contact all pins in the assembly.

STATEMENTS OF INVENTION

According to the present invention there is provided a process for theelectrochemical dissolution of a metallic structure having a pluralityof electrically conducting components, the process comprising utilisingthe structure as a sacrificial electrode in an electrochemical cell soas to dissolve at least a part of the structure, characterised in that,prior to use of the structure as a sacrificial electrode, molten metalis allowed to solidify about the structure so as electrically to connecttogether said components.

In the case where the metallic structure is a fuel assembly, one end ofthe assembly is lowered into a vessel containing molten metal. Themolten metal is allowed to cool as a result of which a block of metal iscast around the end of the fuel pins held in the assembly. The fuelassembly is then lowered into the electrolyte liquid contained within anelectrochemical cell with the cast metal block uppermost. During theoperation of the electrochemical cell, the fuel pins dissolve into theelectrolytic liquid and, as this happens, the fuel assembly may befurther lowered into the cell until it is essentially all consumed.

The metal block may be formed from stainless steel which is the materialused at that end of the fuel assembly having guide nozzles forregulating water flow through the assembly. The block of metal maytypically extend for a length of about 10 cms.

By utilising a cast metal block at one end of the fuel assembly,electrical contact can then be simply made to the cast block itselfwhich in turn connects to each fuel pin. In general the material usedfor casting must be electrically conducting. It should also, in theparticular case of oxidised fuel assemblies, melt at a high enoughtemperature for the diffusion of oxygen from the oxide layer on theoutside of the fuel pins into the molten melt to occur at a fast enoughrate. However the melting temperature should not be so high thatembrittlement or melting of the cladding itself takes place.

In a particular embodiment of the invention stainless steel is melted ina graphite crucible to achieve the appropriate balance of meltingtemperature and reducing conditions since some of the graphite dissolvesin the stainless steel giving it a “reducing” nature. In anotherembodiment, a higher carbon content steel is used, thereby removing theneed for a graphite furnace.

The temperature to which the molten metal is raised is chosen to givegood fusion bonding between the metal structure and the molten metal butwith the avoidance of a temperature which is so high that embrittlementtakes place or, in extreme cases, penetration of the metal structure bythe molten metal. In the case where the molten metal is stainless steelwhich is melted in a graphite crucible and the metal structure is a fuelassembly including Zircaloy cladding, the temperature was between 1350°and 1420° C. preferably between 1375° and 1395° C. and most preferablyabout 1385° C.

Preferably, the metallic structure is substantially fully immersed intothe molten metal prior to solidification thereof. Preferably, the moltenmetal is then cooled at a rate of at least 50° C. min⁻¹, more preferablyat least 100° C. min⁻¹ and most preferably about 200° min⁻¹.

DESCRIPTION OF PREFERRED EMBODIMENTS

A number of commissioning tests were performed to ensure that thefurnaces used, which are radio frequency (RF) furnaces, functioncorrectly and there is compatibility between the Zircaloy cladding andthe molten stainless steel. In addition commissioning tests alsoinvolved resistance measurements on lengths of cladding which had beenoxidised in air to produce an oxide layer having a thickness ofapproximately 20 μm. The tests showed that the stainless steel startedto melt at approximately 1250° C., that is to say, at a considerablylower temperature than its specified melting point range of 1400-1455°C.

The tests also demonstrated that the high resistance of the zirconiumoxide (>2 MΩ) is eliminated, the resistance dropping to 0.1Ω before thestainless steel begins to melt.

A test carried out in a graphite crucible without the presence ofstainless steel revealed that zirconium oxide is reduced on heating in apure argon/carbon monoxide atmosphere.

In addition the tests demonstrated that Zircaloy reacts strongly withmolten stainless steel at 1500° C. resulting in penetration of thestainless steel melt inside the Zircaloy cladding and severe interactionbetween the two alloys. The microstructure of both the stainless steeland the Zircaloy cladding is altered. The steel structure consisted ofaustenite grains in a complex eutectic mixture with inclusions ofgraphite flakes. The Zircaloy structure had an α-Zr layer in the outersurface and columnar grains at the remaining inside thickness. There wasalso a layer of intermetallic compounds at the Zircaloy/stainless steelinterface. In some tests the cladding had cracked at the stainless steelminiscus region due to the complex interaction in molten stainlesssteel, that is to say, differential contraction between the two alloyson cooling and hoop stresses generated in the Zircaloy cladding as aresult of oxygen diffusion to form an α-Zr layer. The shape of theZircaloy cladding within the solidified steel was no longer round butheavily convoluted and the cladding was reduced in thickness.

Tests were carried out at 1255, 1310, 1358 and 1415° C. to determine theoptimum temperature for reduction of the zirconium dioxide and todevelop a good fusion bond between the Zircaloy and the stainless steel.The results showed that, although the resistance had dropped to aminimum value (approximately 0.02Ω) in all cases, there was a lack offusion bonding at temperatures of 1255 and 1310° C. The tests performedat 1358 and 1415° C. show both elimination of the oxide layer and goodfusion bonding between the two alloys. Accordingly, the optimumtemperature was selected to be about 1385° C. (midway between 1358 and1415° C.) to ensure good fusion bonding and minimised embrittlement ofthe cladding.

Further tests were performed on single cladding lengths with differingoxide thicknesses (15, 19, 36 and 42 μm) at 1385° C. In all cases thehigh resistance due to the zirconium dioxide layer dropped to 0.1Ωbefore the stainless steel began to melt. Subsequent to the stainlesssteel melting, there was a very small further drop in resistanceprobably due to dissolution of the residual oxide layer on the Zircaloyclad surface. A minimum resistance of around 0.025Ω was attained in allcases before the temperature was raised to 1385° C. There was nosystematic relationship between the oxide thickness and the temperatureto achieve a minimum; even the cladding with the maximum oxide thickness(42 μm) achieved the minimum resistance at a similar temperature. Avisual metallographic examination of the samples after the testsindicated complete melting of the stainless steel and good fusionbonding between the Zircaloy and the stainless steel.

In a further test a small model fuel assembly (comprising a 3×3 matrixof Zircaloy cladding lengths) was heated to a melt temperature of about1385° C. Movement of the rod assembly (supporting the nine claddinglengths) to one side of the graphite crucible occurred at about 1300° C.indicating the bulk melting of the stainless steel block. A rapid dropin resistance from above 2 MΩ to less than 0.1Ω occurred in thetemperature range between 1000 and 1260° C. Subsequently the resistancecontinued to drop very slowly with temperature until a minimum value of0.025Ω was recorded at 1350° C. There was no further drop in resistanceduring heating to the target temperature of 1385° C. After the test, itwas found that the steel had melted around all the Zircaloy lengths andthere was good contact between the two alloys. One cladding length hadbroken near the meniscus region and two lengths showed two cracks in thesame region, the rest remaining intact.

In order to limit embrittlement of the cladding melt interface, theexperimental technique was modified to hold the cladding partiallyimmersed in the stainless steel block during the heating and meltingprocess. Immediately before cooling begins the cladding is fullyimmersed into the melt. In this way the top length of the cladding withlimited embrittlement is encased by the stainless steel cast, thusreducing the tendency to fracture at the gas/melt interface. The minimumresistance criterion is met by the lower part of the Zircaloy cladding,which is fully fused in the stainless steel.

Two Tests were Carried Out:

Test 1: Heating to 1370° C. at 200° min⁻¹, hold time 2 minutes andcooling at ˜200° C. min⁻¹ ie furnace turned off.

Test 2: Heating to 1370° C. at 200° C. min⁻¹, hold time 2 minutes andcooling at 50° C. min⁻¹.

In order to allow partial immersion of the cladding during the heatingand melting process and subsequent full immersion at the targettemperature, some alterations were made to the RF furnace components.The stainless steel block was redesigned ie a 38 mm deep bore wasdrilled in the top length to immerse the cladding partially during theheating and melting process and the bottom 12 mm length was a solidblock to allow the full immersion (further immersion by 10 mm) aftermelting. The graphite cubicle was dished at the top part to accommodatethe molten metal ejected during the full immersion. The top plate of thefurnace was provided with double seal entry to allow the gas tightmovement of the cladding during the full immersion of the cladding atthe final stage.

The experimental technique involved heating the Zircaloy tube having anoxide layer ˜30 μm, and stainless steel contained in a graphite crucibleusing a R F furnace. As open-ended tubes were provided, a pluggingdevise (alumina pellet) is inserted in the bottom of the tubes tominimise entry of the stainless steel melt during the test. Theconductivity across the cladding/melt interface is measured in eachcase. The melt temperature (graphite inside temperature) was monitoredby a thermocouple located inside a closed end alumina tube, insertedinto a bore drilled in the crucible wall thickness just next to thestainless steel surface. Another similar thermocouple was set up tomeasure the temperature near the bottom end of the stainless steel block(at a mid position of 12 mm long solid stainless steel end). The insideof the cladding is flushed with helium gas during each test in order toexclude any residual oxygen.

For both tests the resistance had decreased to a lower value of 0.1Ωbefore melting of the stainless steel had initiated. The decrease inresistance occurred very rapidly in the temperature range of 1150 to1300° C. Below this temperature range the ZrO₂ resistance was out of theresistance measurement range (>2 MΩ), and the system produced erraticresistance values. In both the tests gas, possibly CO/CO², was observedsparging through the molten stainless steel, and this effect was morepronounced and lasted longer for test 2. There were no surface cracks atthe gas/melt interfaces or failure of the Zircaloy cladding during posttest handling. However, a longitudinal crack appeared after test 1, 20mm below the gas/melt interface; and a circumferential crack after test2, 10 mm below the gas/melt interfaces (at the sites of cavities in thestainless steel cast). No stainless steel was observed inside theZircaloy tubes after the tests.

In test 1, the resistance dropped to 0.1Ω at 1275° C. and continued todrop very slowly with temperature until a minimum value of 0.032Ω wasrecorded at the target melt temperature (˜1370° C.). The resistancevalue stayed almost constant on cooling the sample to room temperature.

After the test, it was found that the steel had melted around theZircaloy cladding and there was good contact between the two materials.There was no sign of cracking in the region of the gas/melt interface.The ejection of molten metal onto the dished part of the graphitecrucible during the full immersion of the cladding indicated that thestainless steel had been fully molten.

In test 2, the resistance dropped to 0.1Ω at 1290° C. and continued todrop very slowly with temperature until a minimum value of 0.025Ω wasrecorded at the target melt temperature. The resistance value increasedslightly to 0.029Ω on cooling the sample to room temperature.

After the test, it was found that the steel had melted around theZircaloy cladding and there was good contact between the two materials.There was no sign of cracking in the region of the gas/melt interface.The ejection of molten metal onto the dished part of the graphitecrucible during the full immersion of the clad indicated that thestainless steel had been fully molten.

The samples for metallographic examination from each test were selectedfrom two different positions; a transverse section at a position on thetip of the inner thermocouple and a longitudinal section at the gas/meltinterface. The results of transverse sections show that there was goodfusion bonding between the Zircaloy cladding and stainless steel (inboth cases); however, in some areas (especially in the case of test 2)the contact between the two was lost due to the cavities in the melt.The photomicrographs show that the microstructure of the solidifiedstainless steel consisted of light and dark phases in the form ofacicular (both light and dark phases) and polygonal (usually darkphases) grains. Coarse graphite flakes were also found within themixture of the light and dark phases. The graphite flake size depends onthe cooling rate, the faster the cooling rate the smaller the size. TheZircaloy had recrystallised with single grains traversing the claddingwall. The recrystallised grains also seemed to have a eutectic phasebetween them.

The micrographs show three distinctive layers across the stainless steelZircaloy interface; a thin layer (˜5 μm thick) next to the stainlesssteel surface, an adjacent smooth layer (α-Zr(O) (˜90 μm thick) growninto the Zircaloy matrix and an intermetallic layer (˜15 μm thick) atthe α-Zr(O)/Zircaloy interface.

The results of longitudinal sections show that there was a lack offusion bonding between the Zircaloy cladding and stainless steel (inboth cases, in a small depth studied up to 3.5 mm) near the gas/meltinterface due to the presence of unreduced ZrO² at the stainlesssteel/Zircaloy interface. The presence of ZrO₂ was patchy and to alesser extent in test 2 (than in test 1) which seems to be due to theslow cooling rate and hence the longer reaction time. However, theresultant improvement in embrittlement was better in test 1 as evidencedby topography of tiny cracks; the cracks being wider and longer in thecase of test 2 than in test 1 at the Zircaloy cladding surface aroundthe gas/melt interface.

As has been indicated above, the present invention has application tothe electric chemical dissolution of any metallic object, in particularto metallic assemblies where it is difficult to ensure good electricalconduct to all the parts of the assembly.

What is claimed is:
 1. A process for the electrochemical dissolution ofa metallic structure having a plurality of electrically conductingcomponents, the process comprising utilising the structure as asacrificial electrode in an electrochemical cell so as to dissolve atleast a part of the structure, characterised in that, prior to use ofthe structure as a sacrificial electrode, molten metal is allowed tosolidify about the structure so as electrically to connect together saidcomponents.
 2. A process according to claim 1 in which the metallicstructure is a spent nuclear fuel assembly.
 3. A process according toclaim 2 in which the molten metal is allowed to solidify about one endof the assembly and the assembly is then used as a sacrificial electrodewith that end positioned uppermost and the other end extending into thecell electrolyte.
 4. A process according to claim 2 in which the fuelassembly includes a plurality of pins each being coated with an oxidefilm.
 5. A process according to claim 2 in which the fuel assemblyincludes a plurality of Zircaloy clad pins.
 6. A process according toclaim 1 in which the molten metal is stainless steel.
 7. A processaccording to claim 6 in which the molten metal is at a temperaturebetween 1350° and 1420° C.
 8. A process according to claim 7 in whichthe molten metal is a temperature between 1375° and 1395° C.
 9. Aprocess according to claim 7 in which the molten metal is a temperatureof the order of 1385° C.
 10. A process according to claim 1 in which themetallic structure is substantially fully immersed into the molten metalprior to solidification thereof.
 11. A process according to claim 10 inwhich, after said full immersion, the molten metal is cooled at a rateof at least 100° C. min⁻¹.